U.S. patent number 6,901,654 [Application Number 09/757,909] was granted by the patent office on 2005-06-07 for method of fabricating a coil and clamp for variable reluctance transducer.
This patent grant is currently assigned to Microstrain, Inc.. Invention is credited to Steven W. Arms, Michael J Hamel, Steven Ward Mundell, Christophor Pruyn Townsend.
United States Patent |
6,901,654 |
Arms , et al. |
June 7, 2005 |
Method of fabricating a coil and clamp for variable reluctance
transducer
Abstract
Improved coils and clamps for variable reluctance sensors are
disclosed. A method of fabricating a discrete coil involves
providing a conductor wound in a coil on a tube. The coil has a
coil outer surface that has insulation. A window is opened in the
insulation on the coil outer surface to expose conductor of the
coil for a contact. A movable core is provided within the tube for
adjusting inductance of the coil. In one embodiment, the coil and
tube are diced into small coils after the windows for each coil are
opened. Another aspect the invention is a clamp comprising an
elastic material, a shape memory alloy, and an apparatus for
activating the shape memory alloy. The clamp holds the moveable
core in its peak position. When the alloy is activated it changes
shape and provides a force on the elastic material to change
clamping state for resetting the transducer. The coils and clamps
can be used for a variety of purposes in addition to the variable
reluctance sensors.
Inventors: |
Arms; Steven W. (Williston,
VT), Hamel; Michael J (Williston, VT), Mundell; Steven
Ward (S. Burlington, VT), Townsend; Christophor Pruyn
(Shelburne, VT) |
Assignee: |
Microstrain, Inc. (Williston,
VT)
|
Family
ID: |
26870650 |
Appl.
No.: |
09/757,909 |
Filed: |
January 10, 2001 |
Current U.S.
Class: |
29/606;
219/121.7; 29/412; 29/417; 29/602.1; 29/832; 29/840; 336/192;
336/200 |
Current CPC
Class: |
G01B
7/24 (20130101); H01F 21/06 (20130101); H01F
41/04 (20130101); Y10T 29/49789 (20150115); Y10T
29/4913 (20150115); Y10T 29/49798 (20150115); Y10T
29/49144 (20150115); Y10T 29/49128 (20150115); Y10T
29/4902 (20150115); Y10T 29/49073 (20150115) |
Current International
Class: |
G01B
7/16 (20060101); G01B 7/24 (20060101); H01F
21/02 (20060101); H01F 21/06 (20060101); H01F
41/04 (20060101); H01F 005/02 (); H01F
003/06 () |
Field of
Search: |
;29/605,606,592.1,594,595,593,602.1,412,417,832,840,860 ;324/667
;336/192,200,96 ;340/870.33,870.31,870.35 ;73/786
;219/121.7,121.71 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
60-124487 |
|
Jul 1985 |
|
JP |
|
1-179406 |
|
Jul 1989 |
|
JP |
|
Primary Examiner: Tugbang; A. Dexter
Attorney, Agent or Firm: Leas; James Marc
Parent Case Text
This application claims the benefit of Provisional application Ser.
No. 60/174,903, filed Jan. 10, 2000.
Claims
What is claimed is:
1. A method of fabricating an electronic device, comprising the
steps of: a) providing a coil of conductor and an insulation, said
coil of conductor having a coil outer surface and a coil inner
surface, said insulation on said coil outer surface, said coil of
conductor further comprising a coil length; b) forming openings in
portions of said insulation on said coil outer surface and exposing
conductor in said openings for external contacts; and c) dicing
completely through said coil to provide a plurality of short coils,
wherein each said short coil has at least one said opening in said
insulation, wherein each of said plurality of short coils has a
short coil length that is less than said coil length.
2. The method as recited in claim 1, further comprising the steps
of: e) providing a substrate; and f) surface mounting said coil to
said substrate.
3. The method as recited in claim 2, wherein, in said providing
step (e), said substrate comprises a printed circuit board, a
ceramic substrate, a flexible material, or an integrated
circuit.
4. The method as recited in claim 2, wherein said surface mounting
step (f) comprises die step of electrically connecting conductor
exposed in said opening in said insulation to said substrate.
5. The method as recited in claim 4, further comprising the step of
providing a solder or conductive polymer, wherein said electrical
connecting step comprises joining with said solder or said
conductive polymer.
6. The method as recited in claim 5, wherein said joining step
comprises providing solder paste between said substrate and said
conductor exposed in said opening and heating to reflow said
solder.
7. The method as recited in claim 2, further comprising the step of
mounting additional electronics on said substrate.
8. The method as recited in claim 7, further comprising the step of
connecting said additional electronics to said coil.
9. The method as recited in claim 8, further comprising the step of
providing a housing for holding said coil, said substrate, and said
additional electronics.
10. The method as recited in claim 9, further comprising the step
of hermetically scaling said housing.
11. The method as recited in claim 9, further comprising the step
of providing pins for external connection through said housing.
12. The method as recited in claim 9, wherein said coil and said
additional electronics comprise a sensor.
13. The method as recited in claim 12, wherein said sensor
comprises a variable reluctance transducer.
14. The method as recited in claim 12, wherein said sensor is for
measuring strain, displacement, acceleration, force, or
pressure.
15. The method as recited in claim 12, further comprising the step
of providing a circuit to correct for temperature variation.
16. The method as recited in claim 15, wherein said circuit is
integrated within said housing.
17. The method as recited in claim 15, wherein said circuit is
located within signal conditioning electronics separate from said
housing.
18. The method as recited in claim 7, wherein said additional
electronics provides excitation or synchronous demodulation.
19. The method as recited in claim 7, wherein said additional
electronics converts an ac waveform to a dc voltage.
20. The method as recited in claim 1, further comprising the step
of enclosing said coil in a housing and hermetically sealing said
housing.
21. The method as recited in claim 1, wherein said step of forming
openings in portions of said insulation comprises laser ablating
said insulation.
22. The method as recited in claim 21, wherein said step of laser
ablating said insulation, comprises directing light from a laser on
said insulation.
23. The method as recited in claim 22, wherein said laser comprises
an excimer laser.
24. The method as recited in claim 21, wherein said coil comprises
a plurality of turns of said wire and wherein said step of laser
ablating said insulation comprises opening said insulation over a
plurality of said turns of wire.
25. The method as recited in claim 21, wherein said step of laser
ablating said insulation comprises ablating a ring shaped opening
in said insulation.
26. The method as recited in claim 1, wherein said insulation
comprises polyimide.
27. The method as recited in claim 1, wherein said step of forming
openings in portions of said insulation comprises abrading said
insulation.
28. The method as recited in claim 1, wherein said step of forming
openings in portions of said insulation comprises chemically
etching said insulation.
29. The method as recited in claim 1, further comprising the step
of providing a tube, said tube having an outer surface and an inner
surface, wherein said providing step (a) comprises providing said
coil inner surface and said insulation on a said tube outer
surface.
30. The method as recited in claim 29, wherein said providing step
(a) comprises the step of providing a wire, and winding said wire
around said tube.
31. The method as recited in claim 30, wherein said wire comprises
an insulated wire and said step (a) comprises winding said
insulated wire around said tube.
32. The method as recited in claim 30, wherein, in said providing
step (a), said wire comprises two ends, wherein neither of said
ends extends from said coil for contacting.
33. The method as recited in claim 29, further comprising the steps
of providing a movable core within said tube inner surface and
moving said movable core within said tube inner surface for
adjusting inductance of said coil.
34. The method as recited in claim 35, further comprising the step
of enclosing said coil in a housing and hermetically sealing said
housing.
35. The method as recited in claim 33, wherein said step of forming
said openings in portions of said insulation comprises laser
ablating said insulation.
36. The method as recited in claim 35, wherein said coil comprises
a plurality of turns of said wire and wherein said step of laser
ablating said insulation comprises opening said insulation over a
plurality of said turns of wire.
37. The method as recited in claim 35, wherein said step of laser
ablating said insulation, comprises directing light from a laser on
said insulation.
38. The method as recited in claim 37, wherein said laser comprises
an excimer laser.
39. The method as recited in claim 35, wherein said step of laser
ablating said insulation comprises ablating a ring shaped opening
in said insulation.
40. The method as recited in claim 33, further comprising the step
of providing a structure for holding position of said core within
said tube.
41. The method as recited in claim 40, further comprising the step
of providing a structure for resetting position of said core within
said tube.
42. The method as recited in claim 41, wherein said structure for
resetting position of said core within said tube comprises an
electronically controllable clamp.
43. The method as recited in claim 42, wherein said electronically
controllable clamp comprises a shape memory alloy.
44. The method as recited in claim 41, wherein said structure for
resetting position of said core further comprises a spring so said
core can snap to a new position when said clamp is released.
45. The method as recited in claim 29, further comprising the steps
of: e) providing a substrate; and f) surface mounting said coil to
said substrate.
46. The method as recited in claim 45, wherein said providing step
(e), said substrate comprises a printed circuit board, a ceramic
substrate, a flexible material, or an integrated circuit.
47. The method as recited in claim 45, wherein said surface
mounting step (f) comprises the step of electrically connecting
conductor exposed in said opening in said insulation to said
substrate.
48. The method as recited in claim 47, further comprising the step
of providing a solder or conductive polymer, wherein said
electrical connecting step comprises joining with said solder or
said conductive polymer.
49. The method as recited in claim 48, wherein said joining step
comprises providing solder paste between said substrate and said
conductor exposed in said window and heating to reflow said
solder.
50. The method as recited in claim 45, further comprising the step
of mounting additional electronics on said substrate.
51. The method as recited in claim 50, wherein said additional
electronics converts an ac waveform to a do voltage.
52. The method as recited in claim 50, further comprising the step
of connecting said additional electronics to said coil.
53. The method as recited in claim 52, further comprising the step
of providing a housing for holding said coil, said substrate, and
said additional electronics.
54. The method as recited in claim 53, further comprising the step
of hermetically sealing said housing.
55. The method as recited in claim 53, further comprising the step
of providing pins for external connection through said housing.
56. The method as recited in claim 53, wherein said coil and said
additional electronics comprise a sensor.
57. The method as recited in claim 56, wherein said sensor
comprises a variable reluctance transducer.
58. The method as recited in claim 56, wherein said sensor is for
measuring strain, displacement, acceleration, force, or
pressure.
59. The method as recited in claim 56, further comprising the step
of providing a circuit to correct for temperature variation.
60. The method as recited in claim 59, wherein said circuit is
integrated within said housing.
61. The method as recited in claim 59, wherein said circuit is
located within signal conditioning electronics separate from said
housing.
62. The method as recited in claim 50, wherein said additional
electronics provides excitation or synchronous demodulation.
63. A method of fabricating an electronic device, comprising in
order, the steps of: a) providing a coil of conductor and an
insulation, said coil of conductor having a coil outer surface and
a coil inner surface, said insulation on said coil outer surface,
said coil of conductor further comprising a coil length; b) forming
openings in portions of said insulation on said coil outer surface
and exposing conductor in said openings for external contacts; c)
dicing through said coil to provide a plurality of short coils,
wherein each said short coil has at least one said opening in said
insulation, wherein each of said plurality of short coils has a
short coil length that is less than said coil length; d) providing
a substrate; e) surface mounting said coil to said substrate; f)
mounting additional electronics on said substrate; g) connecting
said additional electronics to said coil; and h) providing a
housing for holding said coil, said substrate, and said additional
electronics.
64. A method of fabricating an electronic device, comprising in
order, the steps of: a) providing a coil of conductor, an
insulation, and a tube, said coil of conductor having a coil outer
surface and a coil inner surface, said insulation on said coil
outer surface, wherein said tube has a tube outer surface and a
tube inner surface, and wherein said coil of conductor and said
insulation are on said tube outer surface, further wherein said
coil of conductor further comprises a coil length; b) forming
openings in portions of said insulation on said coil outer surface
and exposing conductor of said coil for contacts; c) dicing through
said coil to provide a plurality of short coils, wherein each said
short coil has at least one said opening in said insulation,
wherein each of said plurality of short coils has a short coil
length that is less than said coil length; and d) providing a
movable core within said tube and providing for moving said movable
core within said tube for adjusting inductance of said coil.
Description
FIELD OF THE INVENTION
This invention generally relates to sensors. More particularly, it
relates to a microminiature, differential variable reluctance
transducer capable of peak strain detection. Even more
particularly, it relates to an improved coil, clamp, and package
for such a peak strain detector, and an improved method of
manufacturing a coil.
BACKGROUND OF THE INVENTION
Civil and military structures, such as buildings, dams, and bridges
can benefit from smart sensors that can report the peak strain to
which they have been exposed. Strain to such structures may peak
when power is not available, such as during hurricanes, tornadoes,
and earthquakes. Devices that do not need power that can measure
and record the extent of strain that may have occurred at these
times are especially important. Similarly, structures that are
repeatedly stressed, such as helicopter and aircraft landing gear,
and which may need to be repaired, reinforced, or replaced if
strain exceeds a threshold, could benefit from strain monitoring,
but these structures are difficult and expensive to permanently
instrument by hard wires to a data recorder. Sensitive instruments
may also be subject to rough handling during transport, but data
recorders powered by batteries may not be a practical or cost
effective monitoring solution for such instruments. Similarly,
recording details of what happens in a vehicular collision or in a
collision between athletes can be of substantial value, and
advantage here is especially great for a device that does not
require a source of power for making its measurement. In addition
to sensors for peak displacement and strain detection, it is
desirable to also provide sensors for peak acceleration, force,
pressure, and torque.
Commonly assigned U.S. Pat. No. 5,777,467 to Arms, et al, ("the
'467 patent") incorporated herein by reference, describes a novel
ultra-miniaturized differential variable reluctance transducer
assembly encased in stainless steel. The assembly contains a free
sliding, magnetically permeable core and two coils surrounding the
core. A split ring mounting adapter system allows for a variable
gauge length and interchangeable mounting pins. A highly flexible
core carrier tube and support wire allows for significant bending
without failure, does not interfere with the coils detection of the
core, and protects the core from corrosion. A sleeve strain relief
sheath has been incorporated with the sensor to avoid excessive
strain to lead wires during and after installation. The position of
the core is detected by measuring the coils' differential
reluctance and transmitted by means of wires or telemetry to
measuring equipment. However, while the '467 patent is suitable for
differential strain detection, it does not provide for holding a
peak strain reading.
Commonly assigned patent application Ser. No. 09/259,615 to Arms,
et al, ("the '615 application") incorporated herein by reference,
describes a passive peak strain detector that is especially useful
in circumstances where power is unavailable. The patent application
demonstrated that tiny, peak strain detection devices with high
strain resolution are fabricated using a differential variable
reluctance transducer with an entrapment collar that provides a
circumferential load to the core to constrain it from free sliding,
holding the peak strain measurement for reading at a later time. No
power is required for making the reading. However, tiny coils, such
as those used for the variable reluctance sensors described in both
the '467 patent and in the '615 application, have windings with
delicate wires that are difficult to handle and difficult to
interconnect with other circuitry.
In U.S. Pat. No. 4,759,120 to Bernstein ("the '120 patent"),
discloses a coil wound around a core. Insulation is removed from
the wire either during or after winding at predetermined locations
to match the location of connection pads in a conductive pattern on
a substrate. The coil is appropriately aligned and laid down on the
substrate and an attachment technique is used to form an electrical
connection between the exposed areas of wire and connection pads on
the substrate. The '120 patent provides for varying the inductance
of the coil so formed by controlling the location of openings in
the insulation. However, the '120 patent provides no way of varying
the inductance of the coil after mounting to the substrate.
Similarly, the spring loaded entrapment collar disclosed in the
'615 application has no adjustment, and the very large force
required to reset the device makes reuse impractical.
Thus, a better system for manufacturing, handling, and using coils
and clamps is needed to provide lower cost variable reluctance
sensors that can detect and hold a peak reading, and clamps that
can be reset, and these solutions are provided by the following
invention.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a
scheme for making contact to a coil that does not rely on
contacting ends of delicate wires while providing for variable
reluctance after mounting on a substrate;
It is a further object of the present invention to provide a robust
scheme for manufacturing and contacting coils that is more reliable
and also much less expensive to fabricate;
It is a further object of the present invention to provide a
movable core for a coil with windows in insulation that allows for
surface mounting the coil on a printed circuit board or other
substrate;
It is a further object of the present invention to provide a clamp
that uses a shape memory alloy to provides a controllable force for
holding position of the core within the coil;
It is a further feature of the present invention that connection is
made through windows opened in insulation on a surface of a coil
wrapped on a tube and having a moveable core inside the tube;
and
It is an advantage of the present invention that a sensor is lower
cost and more robust as a result of having a surface mountable coil
with a moveable coil that provides variable reluctance and a shape
memory alloy clamp for holding peak displacement of the core.
These and other objects, features, and advantages of the invention
are accomplished by a method of fabricating a discrete coil. The
method involves providing a conductor wound in a coil on a tube.
The coil has a coil outer surface. The coil outer surface has
insulation. A window is opened in the insulation on the coil outer
surface to expose conductor of the coil for a contact. A movable
core is provided within the tube for adjusting inductance of the
coil.
Another aspect the invention is also a method of fabricating a
discrete coil. This method involves providing a conductor wound in
a coil. The coil has a coil outer surface. The coil outer surface
has insulation. A plurality of windows are opened in the insulation
on the coil outer surface to expose conductor of the coil for
contacts. The coil is diced into a plurality of short coils. Each
short coil has at least one window in the insulation.
Another aspect of the invention is a discrete winding, comprising a
conductor wound in a coil on a tube. The coil has a coil outer
surface. The coil outer surface has insulation. A window in the
insulation exposes the conductor of the coil for a contact to the
conductor. A movable core is within the tube for adjusting
inductance of the coil.
Another aspect of the invention is a clamp comprising an elastic
material, a shape memory alloy, and an apparatus for activating the
shape memory alloy. When the alloy is activated it changes shape
and provides a force on the elastic material to change clamping
state.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and other objects, features, and advantages of the
invention will be apparent from the following detailed description
of the invention, as illustrated in the accompanying drawings, in
which:
FIG. 1a is a top view of a coil of the present invention;
FIG. 1b is a cross sectional view of the coil;
FIG. 2a is a top view of the coil of FIG. 1 a mounted on a printed
circuit board;
FIG. 2b is a cross sectional view of the coil and printed circuit
board;
FIG. 3 is an oblique view of the coil and a hermetic seal mounted
on the printed circuit board;
FIG. 4 is an oblique view of the hermetically sealed sensor and
core;
FIG. 5a is a side view of the hermetically sealed sensor showing
hermetic seals;
FIG. 5b is a bottom view of the hermetically sealed sensor of FIG.
5a;
FIG. 5c is a top view of the hermetically sealed sensor of FIG.
5a;
FIG. 6 is an oblique view of a clamp of the present invention;
FIG. 7a is an oblique view showing the clamp of the present
invention integrated with the sensor in its initial position;
FIG. 7b is an oblique view showing the clamp of the present
invention integrated with the sensor in its position after a force
has been applied that moves the core through the clamp and within
the coil;
FIG. 7c is an oblique view showing the clamp of the present
invention integrated with the sensor in its position after current
has flowed through the shape memory alloy wire to reset the clamp;
and
FIG. 8 is an oblique view of a simulation of clamp of the present
invention showing displacement of the clamp when various currents
flow through the shape memory alloy wire;
DETAILED DESCRIPTION OF THE INVENTION
The present inventors recognized that a coil for their peak strain
detector could be fabricated and handled more efficiently if
contacts are formed on the surface of a coil that is wound on a
hollow tube, avoiding the need to make contact to ends of delicate
wires of the coil, while permitting use of a moveable core within
the tube.
The present inventors also recognized that the surface mount
approach allows a further lowering of the cost of fabricating coils
for their peak strain detectors by integrating steps for many coils
into a step for a single coil: A long coil is be wound. All the
windows needed for contacts are opened, for example by laser
ablation in the long coil. Then the long coil is diced into the
short coils needed for an application, and each is surface mounted
on a substrate using the windows in the insulation for the
contacts. Thus winding of individual short coils is avoided, and
handling reduced, providing higher reliability and lower cost.
The present inventors also found an improved clamp for their peak
strain detector that is far more controllable. The improved clamp
takes advantage of properties of shape memory alloys. The shape of
the shape memory alloy is adjusted by a method such as providing a
current. This exerts a force on elastic clamping members,
controlling the clamping state of the clamp. Thus, a controllable
clamp is provided, and this is used for resetting the displacement
sensor for reuse after data has been read.
Finally, the present inventors recognized that a hermetically
sealed housing provides significant advantage for long term
reliability, particularly in recognition of the fact that in
bridges and other civil structures, sensors with life spans of many
decades are required.
Wire 20 comprising conductor 22 and insulator 24 is wound on long
tube 26 to form long coil 28, as shown in FIGS. 1a, 1b. Conductor
22 is formed of copper, gold plated copper, tungsten, or another
conductor; insulator 24 of a material such as polyimide,
polyurethane Enamel, nylon, polyester, glass fibers, polybutarate,
or polyvinyl; and tube 26 of a non-magnetic material such as a
polymer, ceramic, or glass. Polyimide, polyimide insulated
stainless steel tubing, or fused silica can also be used.
Tube 26 can be quite long, as shown by the top view in FIG. 1a. A 4
foot long coil was fabricated, for example, but much longer or
shorter lengths can be used. Windows 30 in insulator 24 are formed
as rings extending around coil 28, exposing conductor 22' in
windows 30 at locations as required for connection to pads 32 of
substrate 34, as shown in FIGS. 2a and 2b. Windows 30 are formed by
a process such as laser ablating, sand blasting, or masking and
chemical etching that removes affected areas of insulator 24
without damaging conductor 22' thereby exposed. Laser ablation can
be performed using a CO.sub.2 laser to remove polyurethane. An
excimer laser in an argon ambient can be used to ablate polyimide.
Such services may be provided by Resonetics, Nashua, N.H.,
Once windows 30 have been formed along coil 28, coil 28 is diced
into short coils 28', along cutting plane 36, as shown in FIG. 2a.
Winding a long coil and opening windows in the long coil for
contact allows mass production, which reduces cost and complexity
that would be required for performing these steps on short coils.
Forming and using windows 30 for surface mount contact to a
substrate also substantially reduces cost as compared with
contacting ends of delicate wire 22' (not shown).
After dicing, short coil 28' is mounted on and connected to
substrate 34 by soldering exposed conductor 22' in windows 30 of
short coil 28' to pads 32 of substrate 34 to form solder joints 38,
as shown in FIGS. 2a, 2b.
Gold plated 48 gauge copper wire with polyurethane insulation was
found to be compatible with solder reflow temperatures and CO.sub.2
laser ablation. Gold plated copper provides advantage since it
eliminates the need for tin plating, has unlimited shelf life,
reduces winding resistance, improves coil Q factor, and facilitates
direct bonding with commercial solder pastes without adding
substantially to cost.
Pads 32 on substrate 34 can be substantially wider than windows 30
in insulator 24, as shown in FIG. 2a, to facilitate alignment there
between. Surface mounting short coil 28' to substrate 32 is also
facilitated by providing ring shaped windows 30 in insulator 24
since coil 28' can then be mounted to substrate 34 rotated in any
orientation. However, windows 30 can also be provided along one
side of coil 28'; in this case that side must be oriented down on
substrate 34.
Solder paste can be placed on substrate pads 32 before coil 28 is
placed in position. Then after coil 28 is aligned and placed on
substrate 34, the assembly is heated to reflow the solder paste and
then cooled to form solder joints 38. Substrate 32 is a printed
circuit board, integrated circuit chip, ceramic substrate,
multi-layer ceramic, or flexible printed circuit material, such as
polyimide. Coil 28 and tube 26 are diced with a saw, laser, or
mechanical shears.
Pads 32 of substrate 34 are connected to external connection pads
40 with traces 42 formed on substrate 34. External connection pads
40 can be used for testing substrate 34 as well as for soldering
directly to hermetic feed through pins 44 for hermetically sealed
package 46, as shown in FIG. 3 and FIG. 4.
We constructed stainless hermetic package for DVRT 60 using proven
laser sealing techniques, as shown in FIG. 5. Back 62 of DVRT 60
uses (MIL-H-28719) stainless/glass/kovar or
stainless/glass/stainless feedthrough 46' (Electrovac, Salzweg,
Germany), with four pins 44 for solderable termination to flexible,
multistrand, shielded leadwires (not shown). For applications where
a connector is desirable, a miniature circular connector (Lemo
Electronic Connector, Santa Rosa, Calif.) may be combined with a
micro O-ring (Apple Products, Boston, Mass.) for connection to the
pins (not shown).
Silver solder 66 was used to seal feedthrough 46' to stainless
steel shell 68 of DVRT 60, at back 62. At front 70 of DVRT 60, end
cap 72 was welded to both stainless steel coil tube 26 and
stainless steel shell 68 at weld 74. This packaging scheme will
present only stainless steel and glass materials to the
environment, providing an excellent barrier to moisture ingress.
Hermetic sealing was validated with helium leak testing and saline
soak testing. As an alternative, both ends could be welded or both
soldered depending on whether feedthrough 46' is stainless steel or
gold plated kovar. Alternatively shell 68 could be made out of a
polymer. Shell 68 could be injection molded around DVRT 60 and
connectors 44 to provide a hermetic seal, for example.
Short tube 26' has through hole 48 having inside diameter 50
sufficient to accommodate moveable core 52 that can slide within
short tube 26' to adjust the reluctance of short coil 28', as also
shown in FIG. 4. Measurement of the reluctance of short coil 28'
thus gives indication of the displacement of moveable core 52
within short tube 26'. Slender moveable core 52, which may have a
diameter of 20 mils, is fabricated of a flexible tube of
superelastic nickel titanium, which allows sensor 56 to tolerate
significant out-of-axis strains; this tube also contains a slug of
magnetic material. Sensor 56 can be a DVRT or another displacement
sensor.
In operation, a peak differential variable reluctance transducer
(DVRT) is attached to a structure at two points, as described in
the '615 application. Tensile strain in the structure being
measured provides a powerful force causing core 52 to be pulled out
of hole 48 in body 84 of sensor 56. Spring loaded entrapment collar
80 (FIGS. 6, 7a, 7b, 7c, 8) that is integral with body 84 and
applies a circumferential load to core 52. When the powerful force
causing tensile strain in the structure ends, spring loaded
entrapment collar 80 is strong enough to prevent further movement
of core 52. Therefore, peak tensile strains are stored in the
physical location of core 52. This result is achieved without any
electrical power being applied. Thus, the device provides a
physical location for the peak displacement. This location can
later be measured by measuring the inductance of coil 28' which
gives direct indication of the location of core 52 within hole 48
in coil 28', and the displacement of core 52 from a known starting
position. A differential coil arrangement, as shown in FIG. 2a and
FIG. 3, is used to amplify core position and to cancel the effect
of changes in temperature, as described in the '467 patent and in
commonly assigned U.S. Pat. No. 5,497,147 to Arms, et al,
incorporated herein by reference, ("the '147 patent"). Coil 28'
comprises coil 28a' located between solder connectors 38a and 38b
in windows 300a and 30b and coil 28b' located between solder
connectors 38b and 38c in windows 30b and 30c. A temperature
gradient compensation circuit as described in U.S. Pat. No.
5,914,593 to Arms, et al, incorporated herein by reference, ("the
'593 patent"), can also be used. Entrapment collar 80 need not be
circumferential. It can apply force to one side or opposite sides
of core 52, for example.
The inventors further provided two schemes for resetting the peak
strain detector for repeated measurement (using remote electrical
connections) by employing a shape memory actuator. In order to
reset the peak strain detector, a mechanical force is needed that
is greater than the spring entrapment collar's retaining force,
which tends to maintain the core's position.
The first scheme is a shape memory alloy (SMA) spring. When the SMA
spring is allowed to carry an electrical current, its temperature
rises, and if allowed to rise above its transformation temperature,
the spring will begin to expand, and this expansion can be used to
push the peak detect core back into the DVRT coils, resulting in a
(remotely activated) resetting of the device.
The inventors built and tested SMA springs but found that they had
lower than expected force output so more than 3 amps of current
were required for actuation. Less current was required if an
entrapment collar requiring less clamping pressure was used. But
less clamping pressure was undesirable since the core could slip or
lose its peak displacement position especially in an environment
subject to vibration.
A new entrapment collar clamp 80 using SMA wire 82 was designed and
fabricated by the inventors, as shown in FIG. 6. The new design can
actively increase or decrease entrapment clamp force under the
control of the user.
The new entrapment clamp 80 is formed of heat treated 400 series
stainless steel or any other spring material, and is normally in a
closed set to provide a high clamping force on DVRT core 52, as
shown in FIG. 6 and FIGS. 7a-7c. SMA actuator is comprised of
simple SMA wire 82 (Mondotronics, Inc., San Rafael, Calif.) which
shortens upon flow of current through SMA wire 82, as shown in
FIGS. 7a-7c. The shortened SMA wire, in turn, exerts a force
pulling together top portion 80a of stainless steel entrapment
clamp 80 causing its bottom portion 80b to open, as shown in FIG.
8, allowing core 52 to slide.
In use a very high force, such as may be provided by an event such
as a collision, storm, or earthquake, causes a strain in the
structure to which DVRT 84 is connected. This large force causes
movement of DVRT core 52 from the initial position shown in FIG. 7a
to its final position in FIG. 7b. Core 52 is forced through clamped
entrapment collar clamp 80 by the very high force of the event F.
When the event is over, entrapment collar clamp 80 retains DVRT
core 52 in its peak displacement position. No force is available to
move core 52 back from its peak displacement, and its position can
be accurately measured at a later time by determining the change in
reluctance of DVRT 84.
Once the peak displacement measurement has been taken, DVRT 84 can
be reset for further use. The clamping force provided by entrapment
collar clamp 80 on core 52 of DVRT 84 can be reduced or eliminated
by the user, by turning on current I to flow through SMA wire 82,
as shown in FIG. 7c. This causes SMA wire to contract, exerting a
force on stainless steel entrapment collar 80, causing it to open.
Reset spring 86 can now slide core 52 back out to its original
position. Reset spring 86 does not have to be powerful since
clamping force provided by entrapment clamp 80 is temporarily
reduced or eliminated as current flows through SMA wire 82. Once
reset spring 86 has pushed DVRT core 52 back to its initial
position current to SMA actuator wire 82 is turned off. SMA
actuator relaxes, lengthening to its original shape, and the high
clamping force of entrapment collar clamp 80 is restored. DVRT 84
is now reset for another use.
Finite Element Analyses (FEA) of this new design was performed
prior to production in order to refine the design. FIG. 8 indicates
the FEA predicted displacements, showing opening of entrapment
clamp 80 by about 0.1 mm. From these analyses, the inventors
optimized the design and built a functional prototype, as described
here.
Reset performance for the clamp of FIG. 6 was documented by %
return to full closure over 50 trials. The average clamping force
over the 50 trials was measured at 219.9 grams (std. dev.: 35.4
grams), and the average open force was only 0.948 grams (std. dev.:
+/-2.6 grams). The 95% confidence interval (C.I.) for the clamped
forces range from 149-289 grams; while the 95% C.I. for the
unclamped device ranges from 0-6.2 grams. These data show that the
force exerted by the SMA entrapment clamp is much higher than the
unclamped (SMA actuated) force. Therefore, provided that power can
be delivered (temporarily) to generate adequate current (and hence
force) through shrinkage of the SMA wire, a spring with a
relatively light spring constant will reset the peak detect core
reliably. This greatly reduces the potential for the reset spring
of causing inadvertent slippage of the core.
As described in the '147 and '467 patents, the transducer can also
include a transmitter for wireless data transmission.
While several embodiments of the invention, together with
modifications thereof, have been described in detail herein and
illustrated in the accompanying drawings, it will be evident that
various further modifications are possible without departing from
the scope of the invention. Nothing in the above specification is
intended to limit the invention more narrowly than the appended
claims. The examples given are intended only to be illustrative
rather than exclusive.
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